1. Field of the Invention
The invention relates to catalytic reduction of oxides of nitrogen, NO.sub.x, in the presence of carbonaceous materials.
2. Description of Related Art
In Green et al, U.S. Pat. No. 4,828,680, which is incorporated herein by reference, the level of No.sub.x emissions from a fluidized catalytic cracking (FCC) unit was reduced by incorporating carbonaceous particles such as sponge coke or coal into the circulating inventory of cracking catalyst. The carbonaceous particle performed several functions, selectively absorbing metal contaminants in the feed and also reducing NO.sub.x emissions in certain instances.
The patent included data showing that there was negligible reduction of NO by contact of flue gas with either clean sand or clean-burned FCC catalyst. Contacting a flue gas with coked FCC catalyst brought about a significant reduction in NO. Coked FCC catalyst, at 700.degree. C., was able to reduce the NO concentration of an inlet gas mixture from 170 ppm NO to an NO concentration of 84 ppm, a reduction of 51%.
This patent also disclosed a two-stage FCC regenerator wherein flue gas from the second stage of regeneration contacted coked catalyst. The flue gas, containing NO.sub.x from producing a clean-burned catalyst in the second stage was treated by contact with fresh, spent catalyst added to the first stage.
I realized that the use of carbonaceous substances to react with NO.sub.x -containing flue gases provided a powerful way to reduce NO.sub.x emissions. The only drawback to the approach was that the reduction was either slow or only moderately effective. In a properly designed FCC unit, such as that shown in U.S. Pat. No. 4,828,680, slow reaction of NO with carbonaceous materials is not a problem.
Significant NO.sub.x reductions can be achieved by allowing NO.sub.x -containing flue gas to contact spent catalyst in a staged catalyst regeneration. However, the capital cost would be prohibitive unless a grass roots unit were involved.
I wanted a way to speed up the reaction of NO.sub.x with carbonaceous substances. This would permit greater reductions in NO.sub.x emissions to be achieved, or alternatively, allow smaller sized vessels to be used for the contact of NO.sub.x -containing flue gas with carbonaceous materials.
I also wanted to have a process which could be used to clean up NO.sub.x -containing flue gases from any source, not just for FCC units. Because of the widespread use of FCC units, I wanted a process which could be incorporated, at a minimal expense, into existing FCC units, making maximum use of presently installed equipment.
I discovered that the reduction of NO.sub.x emissions by contacting of a NO.sub.x -containing gas stream with a carbonaceous substance could be improved by the addition of a NO.sub.x conversion catalyst. With the use of a NO.sub.x conversion catalyst, the process became one which could be used for stack gas cleanup in general, or for use in FCC units.